1. Field of the Invention
The present invention relates to an active control apparatus, and more particularly to an active control apparatus which can prevent instability such as howling phenomenon, which easily occurs when the active noise control is performed.
2. Description of the Related Art
In order to deal with the noise problem of various types of equipments, an active noise control apparatus to which an acoustic control technique is applied has been recently proposed.
The conventional active noise control apparatus is structured such that sound(noise) emitted from a noise source contained in a duct is prevented from being leaked outside from an opening of the duct. In other words, in this type of the apparatus, noise emitted from the noise source is detected by a microphone, and an output signal of the microphone is introduced to a signal processor with a built-in filter having a predetermined filter coefficient. Then, a secondary sound source, that is, a speaker is operated by the signal obtained by passing through the filter, and noise on the opening of the duct, which is an object to be controlled, is actively canceled by sound emitted from the speaker.
However, the conventional active noise control apparatus has a decisive disadvantage in which instability such as howling phenomenon easily occurs. In other words, sound emitted from the speaker is reflected by the wall of the duct, and detected by the microphone again. More specifically, an electric-acoustic-system feedback circuit is formed in the active noise control apparatus. Accordingly, the apparatus diverges sound depending on the gain, so that sufficient noise reduction effect cannot be obtained.
In order to prevent such a disadvantage, for example, a two-microphone method is proposed in "Active Control of Duct Outlet Emission Sound, by Nishimura, Arai, Shimgaku Giho, vol. 88, No. 105(1988) in Japanese."
According to the above method, sound emitted from the secondary sound source is detected by two microphones, the output signal of one microphone and the output signal output from the other microphone and passed through a delay function circuit with a time delay corresponding to the distance between two microphones, are synthesized by a signal subtracter, and a synthesizing signal is used as a detection signal, so that influence of the secondary sound source signal is removed.
Such a two-microphone method is useful for the long duct structure. However, according to the two-microphone method, sufficient effect to the relative short duct cannot be obtained for the following reason:
More specifically, a sound component P.sub.M, which is detected by two microphones, can be expressed as a sum of two components as shown by the following equation: EQU P.sub.M =P.sub.MS +P.sub.MA ( 1)
wherein P.sub.MS is a component sent only from the noise source, and P.sub.MA is a component from the secondary sound source. Therefore, in order to satisfy a condition of P.sub.MA =0, it is necessary to obtain transfer function G.sub.1 to be set to one of two microphones.
If it is assumed that a distance, which is from the secondary sound source to the microphone positioned at a portion away from the secondary sound source, is L, a distance between two microphones is .DELTA.L, and a sound wave to be canceled is limited to a frequency range, which is relatively longer than a cross sectional mode of the duct, that is, a range, which can be regarded as a plane wave, the following equation can be established referring to FIG. 2, wherein P.sub.PA is progressive wave components from the secondary sound source at a secondary source location, P.sub.rA is retreat wave components thereat, k is the wave number shown by 2.pi.f/c, f: frequency of the secondary sound source, and c: acoustic velocity. EQU P1=P.sub.PA e.sup.jkL +P.sub.rA e.sup.-jkL ( 2) EQU P2=P.sub.PA e.sup.jk(L-.DELTA.L) +P.sub.rA e.sup.-jk (L-.DELTA.L) (3)
where P.sub.1 is an output signal from the microphone 15a away from the secondary sound source, and P.sub.2 an output signal from the microphone 15b near thereto.
P.sub.MA is represented as follows: EQU P.sub.MA =P1-G.sub.1 .multidot.P.sub.2 ( 4)
Substituting the equations (2) and (3) for the equation (4), P.sub.MA is represented as follows: EQU P.sub.MA =e.sup.jkL (1-e.sup.-jk .DELTA.L.multidot.G.sub.1)P.sub.PA +e.sup.-jkL (1-e.sup.jk.DELTA.L .multidot.G.sub.1)P.sub.rA ( 5)
It is necessary to obtain G.sub.1 in which P.sub.MA of the above equation (5) is a howling component and the equation (5) becomes zero.
However, since P.sub.PA and P.sub.rA cannot be directly identified, it is impossible to obtain G.sub.1 in which P.sub.MA =0. For example, in the case that the long duct is used, energy is absorbed by a wall surface during propagation of the components from the secondary sound source. Due to this, P.sub.PA can be set to substantially zero. This case can be expressed by the following equation: EQU G.sub.1 =e.sup.jk.DELTA.L ( 6)
In other words, delay corresponding to the distance between two microphones is given to G.sub.1, so that P.sub.MA can be set to be substantially zero.
However, in the case that the above two-microphone method is employed for a short duct, sound absorption effect cannot be expected during propagation, so that many reflected sound components of the secondary sound are produced. Accordingly, the sound components from the secondary sound source are not completely eliminated. Therefore, even if the active control noise technique of the two-microphone method is applied to a miniaturized product, howling cannot be prevented.